Acceleration sensor

ABSTRACT

A compact, high-sensitivity acceleration sensor that is prevented from being affected by factors other than acceleration, such as a change in temperature, has a bimorph acceleration-sensor element including first and second resonators attached to opposite sides of a base plate with respect to a direction in which acceleration is applied. One longitudinal end of the acceleration-sensor element is fixed such that the first and second resonators bend in the same direction in response to the acceleration. Changes in frequency or changes in impedance in the first and second resonators caused by the bending of the acceleration-sensor element are differentially detected in order to detect the acceleration. The acceleration-sensor element is bendable about a central bending plane in response to the acceleration, the central bending plane being positioned at a central portion of the base plate with respect to the application direction of acceleration. A vibrating section of each of the first and second resonators is disposed close to the fixed end of the acceleration-sensor element.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to acceleration sensors, and particularly,to an acceleration sensor including a piezoelectric material.

2. Description of the Related Art

A known acceleration sensor including piezoelectric ceramics is, forexample, disclosed in Japanese Patent No. 2780594, hereinafter referredto as Patent Document 1. Such an acceleration sensor is provided with abimorph sensor element including a pair of piezoelectric units which arecomposed of piezoelectric ceramics and are integrally joined to eachother in an opposing manner. The sensor element is held inside a casingin a double-supported fashion. When acceleration is applied to theacceleration sensor, the sensor element bends, thus generating stress inthe piezoelectric units. The electric charge or voltage generated due tothe piezoelectric effect is then detected in order to determine theacceleration. Acceleration sensors of this type are advantageous in viewof their compactness and their capability of being formed easily intosurface-mounted units (chip units).

In an acceleration sensor based on the above-described principle, a biascurrent flowing from a circuit is stored in a capacitor C of thepiezoelectric material. In order to prevent the circuit from becomingsaturated, a resistor R is required for allowing the bias current to bereleased. However, since the resistor R and the capacitor C define ahigh pass filter, the acceleration in the frequencies below the cut-offlevel, such as DC and low frequency, cannot be detected.

On the other hand, an acceleration sensor disclosed in JapaneseUnexamined Patent Application Publication No. 2002-107372, hereinafterreferred to as Patent Document 2, particularly, the acceleration sensorshown in FIG. 8 in Patent Document 2, includes a single base plate whoseopposite sides respectively have first and second resonators formed of apiezoelectric material attached thereto so as to form anacceleration-sensor element, each of the first and second resonatorshaving electrodes on opposite sides thereof. One longitudinal end orboth longitudinal ends of the acceleration-sensor element is/are fixedsuch that the first and second resonators are bendable in their opposingdirection in response to acceleration. When the acceleration-sensorelement bends in response to the acceleration, changes in frequency orchanges in impedance in the first and second resonators caused by thebending of the acceleration-sensor element are differentially detectedin order to detect the acceleration.

In this case, the acceleration in a DC or low-frequency level can bedetected. Moreover, the changes in frequency or the changes in impedancein the two resonators are differentially detected instead of beingdetected in a separate manner. This counterbalances the stress (forexample, a stress caused by a change in temperature) applied to bothresonators. Thus, a high-sensitivity acceleration sensor, which isunaffected by, for example, a change in temperature, is achieved.Furthermore, because the central bending plane (i.e., a plane wherestress is 0) is set in the base plate, a large degree of tensile stressand compressive stress can be generated in the resonators disposed onthe opposite sides of the base plate. Accordingly, this improves thesensitivity of the sensor.

Generally, in view of sensitivity, an acceleration sensor having onelongitudinal end of the acceleration-sensor element fixed in acantilever manner is advantageous to an acceleration sensor having bothlongitudinal ends fixed in a double-supported manner. However, in theseacceleration sensors, the vibrating section in each of the first andsecond resonators is positioned at the central portion of the resonatorwith respect to the longitudinal direction thereof. For this reason,signals generated in the first and second resonators in response to theacceleration cannot always be detected efficiently, thus inhibitinghigher sensitivity (S/N ratio) of the sensor.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a compact, high-sensitivityacceleration sensor that is not affected by factors other thanacceleration, such as a change in temperature or other factors.

According to a preferred embodiment of the present invention, anacceleration sensor includes a base plate, and first and secondresonators each formed of a piezoelectric material and each havingelectrodes on two opposite main surfaces thereof, each resonator havinga vibrating section at an intermediate portion of the resonator withrespect to the longitudinal direction thereof. The first and secondresonators are attached to opposite sides of the base plate with respectto a direction in which acceleration is applied so as to define abimorph acceleration-sensor element, wherein one longitudinal end of theacceleration-sensor element is fixed such that the first and secondresonators bend in the same direction in response to the acceleration,and wherein changes in frequency or changes in impedance in the firstand second resonators caused by the bending of the acceleration-sensorelement are differentially detected in order to detect the acceleration.The acceleration-sensor element is bendable about a central bendingplane in response to the acceleration, the central bending plane beingpositioned at a central portion of the base plate with respect to theapplication direction of acceleration. The vibrating section in each ofthe first and second resonators is disposed close to the fixed end ofthe acceleration-sensor element.

According to a preferred embodiment of the present invention, theacceleration-sensor element has a bimorph structure in which theresonators are attached to the opposite sides of the single base plate,and the central bending plane is positioned at the central portion ofthe base plate with respect to the thickness of the base plate.Consequently, when acceleration is applied to the acceleration-sensorelement, the base plate functions as a mass body so as to effectivelygenerate a tensile stress in one resonator and a compressive stress inthe other resonator. In a certain vibration mode, the frequency in theresonator with tensile stress decreases while the frequency in theresonator with compressive stress increases. By differentially detectingthe changes in frequency or the changes in impedance in the resonators,the acceleration can be detected. Moreover, since the changes infrequency or the changes in impedance in the two resonators are detectedin a differential manner, the stress applied to both resonators (forexample, a stress caused by a change in temperature) can becounterbalanced. Accordingly, a high-sensitivity acceleration sensorthat is unaffected by, for example, a temperature change is provided.

In a preferred embodiment of the present invention, the vibratingsection in each of the first and second resonators of theacceleration-sensor element with a cantilever structure is positionedclose to the fixed end of the acceleration-sensor element. In such anacceleration-sensor element with a cantilever structure, the stressgenerated in the first and second resonators in response to accelerationis greater towards the base-end portion. By placing the vibratingsection of each resonator closer to the base-end portion to an extentsuch that the vibration is not interfered, a signal can be detected fromthe base-end portion of the resonator, which is the position where thelargest degree of stress is present. Accordingly, this achieves highersensitivity (S/N ratio) of the sensor.

There are, for example, two approaches for obtaining a signalproportional to the acceleration acting upon the acceleration-sensorelement based on the signals differentially detected from the tworesonators. One approach is to oscillate the first and second resonatorsseparately with different frequencies, determine theoscillating-frequency difference, and obtain the signal proportional tothe acceleration based on the frequency difference. The other approachis to oscillate the first and second resonators with the same frequency,detect the phase difference or the oscillation difference based on thedifference in electric impedance between the resonators, and obtain thesignal proportional to the acceleration based on the phase difference orthe oscillation difference.

Furthermore, a height of the first and second resonators in a directionthat is substantially perpendicular to the application direction ofacceleration is preferably smaller than a height of the base plate inthe direction that is substantially perpendicular to the applicationdirection of acceleration.

Specifically, by reducing the cross-sectional area of the first andsecond resonators, the tensile stress and the compressive stressgenerated in the resonators in response to acceleration are increased,thus further improving the sensitivity (S/N ratio).

Furthermore, the first and second resonators are preferably attached tothe opposite sides of the base plate at positions where the first andsecond resonators are opposed to each other.

Although it is possible to attach the two resonators to the oppositesides of the base plate at positions where the two resonators do notoppose each other, such a structure may lead to detection errors. Indetail, this is due to the fact that if the acceleration-sensor elementbends in response to an external force from a direction other than theapplication direction of acceleration (off-axis bending), the tworesonators may generate different signals. In contrast, by attaching thetwo resonators to the opposite sides of the base plate at positionswhere the two resonators are opposed to each other, signals can bedetected from the two resonators in a differential manner. Thus, thedifference in detection with respect to the off-axis bending can becompensated for.

Furthermore, each of the first and second resonators is preferablyattached to a central portion of the base plate with respect to a heightdirection of the base plate, the height direction being substantiallyperpendicular to the application direction of acceleration.

Consequently, in addition to being attached to the opposite sides of thebase plate at positions where the two resonators are opposed to eachother, each resonator may be attached to the central portion of the baseplate with respect to the height direction. This structure can furthercompensate for the difference in detection since no stress acts upon thetwo resonators in response to off-axis bending.

Furthermore, the base plate and the first and second resonators arepreferably formed of at least one material having substantially the samecoefficient of thermal expansion.

If the coefficient of thermal expansion differs significantly betweenthe base plate and the first and second resonators, a tensile stress ora compressive stress may be generated in the resonators due to a changein temperature in the environment even when no acceleration is applied.This leads to changes in frequency or changes in impedance. By allowingthe base plate and the first and second resonators to have substantiallythe same coefficient of thermal expansion, the temperature drift relatedto the output from the sensor can be prevented, thus reducing thermalhysteresis.

The base plate and the first and second resonators may be formed of thesame material, or may be formed of different materials. The coefficientof thermal expansion between the base plate and the resonators may bedifferent to an extent such that the changes in frequency or the changesin impedance in the resonators in an operating temperature limit arewithin an error range and are thus significantly small.

Furthermore, opposite outer surfaces of the acceleration-sensor elementmay be respectively fixedly supported by a pair of casing components atthe longitudinal end of the acceleration-sensor element, the outersurfaces being opposite to each other with respect to the applicationdirection of acceleration. Moreover, open planes defined by theacceleration-sensor element and the casing components are covered with apair of cover components so that a displacement portion of theacceleration-sensor element, which is bendable in response to theacceleration, is disposed within an enclosed space. Such a packagedstructure allows the displacement portion to be blocked from theoutside, whereby a surface-mounted unit that is prevented from beingaffected by, for example, moisture and dust is provided.

Furthermore, one of the electrodes in each of the first and secondresonators is preferably disposed at a free-end side of the resonatorand is preferably connected with a common electrode via an extractionelectrode provided on the base plate, the common electrode beingprovided at a fixed-end side of an outer surface of a combination of thecasing components and the cover components. Moreover, the otherelectrode in the first resonator is preferably disposed at a base-endside of the first resonator, the electrode being connected with a firstindependent electrode provided at a free-end side of the outer surfaceof the combination of the casing components and the cover components,the electrode being connected with the first independent electrode via afirst extraction electrode provided on one of the casing components. Theother electrode in the second resonator is preferably disposed at abase-end side of the second resonator, the electrode being connectedwith a second independent electrode provided at the free-end side of theouter surface of the combination of the casing components and the covercomponents, the electrode being connected with the second independentelectrode via a second extraction electrode provided on the other casingcomponent.

When using an acceleration-sensor element of a cantilever structure,three electrodes are concentrated at the base-end portion of theacceleration-sensor element, and for this reason, it is difficult to setthese electrodes distant from one another on the outer surface of thepackage. In order to set the three external electrodes distant from oneanother, one pair of the electrodes from the two resonators is connectedto the common electrode, provided at the fixed-end side of the outersurface of the package (the combination of the casing components and thecover components), via the base plate, and the other pair of the tworemaining electrodes is respectively connected to two independentelectrodes, provided at a side of the outer surface opposite to thefixed-end side of the package, via the casing components. Accordingly,when used as a surface-mounted unit, a short circuit is prevented fromoccurring among the electrodes.

Furthermore, the casing components are preferably provided with aplurality of internal electrodes disposed on upper surfaces of thecasing components, the internal electrodes being connected with theelectrodes in each of the first and second resonators.

In this case, the characteristics of the resonators can be measuredeasily by allowing measuring terminals to come into contact with theinternal electrodes disposed on the upper surfaces of the casingcomponents.

Accordingly, preferred embodiments of the present invention provide anacceleration-sensor element having a bimorph structure in whichresonators are attached to opposite sides of a base plate. Whenacceleration is applied to the acceleration-sensor element, changes infrequency or changes in impedance in the resonators are detected in adifferential manner. Accordingly, a high-sensitivity acceleration sensorthat is unaffected by, for example, a temperature change is provided.

Furthermore, the acceleration-sensor element has a cantilever structure,and the vibrating section in each of first and second resonators isdisposed close to a fixed end of the acceleration-sensor element. Thus,a signal can be detected from near the base-end portion, which is wherea large stress is present. This contributes to high sensitivity of thesensor and thus improves the S/N ratio.

These and other features, elements, characteristics and advantages ofthe present invention will become more apparent from the followingdetailed description of preferred embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of an acceleration sensoraccording to a first preferred embodiment of the present invention.

FIG. 2 is an exploded perspective view of the acceleration sensor shownin FIG. 1.

FIG. 3 is an exploded perspective view of an acceleration-sensor elementprovided in the acceleration sensor shown in FIG. 1.

FIG. 4 is a plan view of the acceleration sensor shown in FIG. 1 in astate where cover components of the acceleration sensor are removed.

FIG. 5 includes perspective views illustrating a method for cutting amaster substrate into segments in order to form resonators.

FIG. 6 is a circuit diagram of an example of an acceleration sensordevice provided with the acceleration sensor according to a preferredembodiment of the present invention.

FIG. 7 is circuit diagram of another example of an acceleration sensordevice provided with the acceleration sensor according to a preferredembodiment of the present invention.

FIG. 8 is an exploded perspective view of an acceleration sensoraccording to a second preferred embodiment of the present invention.

FIG. 9 is an exploded perspective view of the acceleration sensor shownin FIG. 8 in a state where cover components of the acceleration sensorare removed.

FIG. 10 is a plan view of the acceleration sensor shown in FIG. 8 in astate where the cover components of the acceleration sensor are removed.

FIG. 11 is an exploded perspective view of an acceleration sensoraccording to a third preferred embodiment of the present invention.

FIG. 12 is an exploded perspective view of the acceleration sensor shownin FIG. 11 in a state where cover components of the acceleration sensorare removed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described.

First Preferred Embodiment

FIGS. 1 to 5 illustrate an acceleration sensor according to a firstpreferred embodiment of the present invention.

An acceleration sensor 1A includes a bimorph acceleration-sensor element2A supported in a cantilever manner by a pair of insulative casingcomponents 6 and a pair of insulative cover components 7 composed of,for example, insulative ceramics. Referring to FIGS. 2 and 3, if thedirection in which acceleration G is applied is defined as the y-axisdirection, the longitudinal and height directions of theacceleration-sensor element 2A are defined as the x-axis direction andthe z-axis direction, respectively.

The acceleration-sensor element 2A in the first preferred embodimentincludes resonators 3 and 4 which are integrally attached to tworespective opposite sides of a base plate 5 with respect to theapplication direction of acceleration (y-axis direction) viacorresponding spacers 51 to 54. The resonators 3 and 4 are resonators ofan energy-trap thickness-shear vibration type and each include apiezoelectric ceramic plate strip. The resonators 3 and 4 respectivelyinclude a pair of electrodes 3 a and 3 b and a pair of electrodes 4 aand 4 b. The electrodes 3 a and 3 b are respectively disposed on upperand lower main surfaces of the piezoelectric ceramic plate strip of theresonator 3, and the electrodes 4 a and 4 b are respectively disposed onupper and lower main surfaces of the piezoelectric ceramic plate stripof the resonator 4, the main surfaces being substantially parallel tothe application direction of acceleration G. One set of the electrodes 3a and 4 a of the resonators 3 and 4 is exposed at the upper side of theacceleration-sensor element 2A, whereas the other set of the electrodes3 b and 4 b is exposed at the lower side of the acceleration-sensorelement 2A. A first-end portion of the electrode 3 a on the uppersurface of the resonator 3 is opposed to a second-end portion of theelectrode 3 b on the lower surface at an intermediate portion of theresonator 3 with respect to the longitudinal direction thereof.Similarly, a first-end portion of the electrode 4 a on the upper surfaceof the resonator 4 is opposed to a second-end portion of the electrode 4b on the lower surface at an intermediate portion of the resonator 4with respect to the longitudinal direction thereof. On the other hand,the second-end portion of the electrode 3 a and the first-end portion ofthe electrode 3 b extend away from each other towards the opposite endsof the resonator 3, and similarly, the second-end portion of theelectrode 4 a and the first-end portion of the electrode 4 b extend awayfrom each other towards the opposite ends of the resonator 4. Theresonators 3 and 4 preferably have substantially the same height H₁ inthe z-axis direction, and the height H₁ is determined based on theresonance frequency of the resonators 3 and 4. Since the height H₁ ofthe resonators 3 and 4 is smaller than a height H₂ of the base plate 5in the z-axis direction, the stress generated in the resonators 3 and 4due to acceleration applied to the resonators 3 and 4 is greater than ina case where H₁=H₂. In the first preferred embodiment, H₁ is preferablyset at about ⅕ or less of H₂.

As shown in FIG. 5, the resonators 3 and 4 are preferably formed bycutting a single master piezoelectric substrate M into segments, andpairing adjacent cut segments so as to form pairs of resonators. Thisreduces the difference in the resonance characteristics including thetemperature characteristics between the resonators of each pair.Accordingly, the difference in the output signal between the tworesonators, which may be caused by a change in temperature, is reducedso as to achieve an acceleration sensor having less output fluctuation.

Even if the resonators 3 and 4 are a pair of adjacent segments cut fromthe same master piezoelectric substrate, there still may be cases wherethe resonance characteristics between the two resonators 3 and 4 aredifferent due to, for example, being attached to the spacers and thebase plate 5. Such different characteristics are output as an outputsignal even when no acceleration is being applied. The set of electrodes3 a and 4 a of the respective resonators 3 and 4 is exposed at one sideof the acceleration-sensor element 2A, and similarly, the set ofelectrodes 3 b and 4 b of the respective resonators 3 and 4 is exposedat the other side of the acceleration-sensor element 2A. Consequently,if the resonance characteristics between the resonators 3 and 4 aredifferent, the electrodes exposed at the upper side or the lower side ofthe acceleration-sensor element 2A may be trimmed using, for example,laser, or may be coated with, for example, frequency-regulation ink sothat the resonance characteristics can be adjusted in order to reducethe difference in the characteristics. Such a trimming process or anink-coating process is performed after an attachment process of thecasing components 6 and a fabrication process of internal electrodes 61,62 b, and 63 b (see FIG. 4). In that case, since measuring terminals cancome into contact with the three internal electrodes 61, 62 b, and 63 bdisposed on the upper surface of the casing components 6, the trimmingprocess can be performed easily while measuring the characteristics ofthe resonators 3 and 4. As a result, a high-precision accelerationsensor with less detection error can be provided.

The upper and lower main surfaces of the resonator 3 are provided withspacers 31 and 32 having the same thickness as the resonator 3. Thespacers 31 and 32 are fixed adjacent to two respective opposite ends ofthe resonator 3 with respect to the longitudinal direction of theresonator 3. Similarly, the upper and lower main surfaces of theresonator 4 are provided with spacers 41 and 42 preferably havingsubstantially the same thickness as the resonator 4. The spacers 41 and42 are fixed adjacent to two respective opposite ends of the resonator 4with respect to the longitudinal direction of the resonator 4. An areawhere the electrodes 3 a and 3 b are opposed to each other and an areawhere the electrodes 4 a and 4 b are opposed to each other definevibrating sections E. In detail, each vibrating section E is disposedwhere the pairs of spacers 31 and 32 or the pairs of spacers 41 and 42are not disposed. In the first preferred embodiment, the spacers 32 and42 disposed adjacent to free ends of the respective resonators 3 and 4have a greater length than the spacers 31 and 41 disposed adjacent tobase ends of the respective resonators 3 and 4. For this reason, asshown in FIG. 3, a distance L2 extending from the vibrating section E ofeach of the resonators 3 and 4 to the free end of theacceleration-sensor element 2A is longer than a distance L1 extendingfrom the vibrating section E to the base end, i.e. a fixed end, of theacceleration-sensor element 2A, such that each vibrating section E isdisposed close to the fixed end. Because a stress generated in responseto acceleration is greater towards the base end of a cantileverstructure, providing the vibrating sections E close to the base ends ofthe resonators 3 and 4 allows the resonators 3 and 4 to receive agreater stress, thus improving the sensitivity of the sensor. The heightof the combination of the resonator 3 and the spacers 31 or 32 and theheight of the combination of the resonator 4 and the spacers 41 or 42are equal to the height H₂ of the base plate 5.

Alternatively, the spacers 31, 32, 41, and 42 may be omitted such thatthe resonators 3 and 4 are directly attached to the two respectiveopposite sides of the base plate 5.

The resonators 3 and 4 are respectively attached to positions on the twoopposite sides of the base plate 5 where the resonators 3 and 4 areopposed to each other, and are most preferably attached to the centralportions of the base plate 5 with respect to the height direction of thebase plate 5. This is due to the fact that even if theacceleration-sensor element were to bend in response to an externalforce from a direction other than the direction in which theacceleration is applied (off-axis bending), the difference in detectionwith respect to the off-axis bending can be compensated for by receivingsignals from the two resonators 3 and 4 in a differential manner. Thedetection difference between the two resonators 3 and 4 opposed to eachother is reduced due to the fact that, even in the case of off-axisbending, the same amount of stress acts upon the two resonators.Moreover, attaching the two resonators 3 and 4 to the central positionsof the base plate 5 with respect to the height direction of the baseplate 5 further reduces the detection difference. Specifically, this isdue to the fact that even when stress is generated in the resonators 3and 4 due to off-axis bending, since each of the resonators 3 and 4bends with respect to a central bending plane disposed at the centralportion thereof in the height direction, the stress is counterbalancedwithin the resonator 3 or 4.

One the side surface of the combination of the resonator 3 and thespacers 31 with respect to the y-axis direction is provided with aconnection electrode 33 connected with the electrode 3 a of theresonator 3 and extending continuously across the side surface in theheight direction (z-axis direction). Similarly, the other side surfaceof the combination of the resonator 3 and the spacers 32 with respect tothe y-axis direction is provided with a connection electrode 34connected with the electrode 3 b of the resonator 3 and extendingcontinuously across the side surface in the height direction (z-axisdirection). On the other hand, one side surface of the combination ofthe resonator 4 and the spacers 41 with respect to the y-axis directionis provided with a connection electrode 43 connected with the electrode4 a of the resonator 4 and extending continuously across the sidesurface in the height direction (z-axis direction). Similarly, the otherside surface of the combination of the resonator 4 and the spacers 42with respect to the y-axis direction is provided with a connectionelectrode 44 connected with the electrode 4 b of the resonator 4 andextending continuously across the side surface in the height direction(z-axis direction). Specifically, the connection electrodes 33 and 43disposed close to the base ends of the resonators 3 and 4, respectively,are disposed on the outer side surface of the combination of theresonator 3 and the spacers 31 and the outer side surface of thecombination of the resonator 4 and the spacers 41.

The base plate 5 is an insulative plate having the same length as theresonators 3 and 4, and is bendable with respect to a central bendingplane (indicated by a dashed line N1 in FIG. 4) in response toacceleration G applied to the acceleration-sensor element 2A. Thecentral bending plane is positioned at the central portion of the baseplate 5 with respect to the thickness direction (y-axis direction) ofthe base plate 5. The base plate 5 and each of the resonators 3 and 4have a gap 5 a therebetween (see FIG. 4) which is given a widerdimension than the range in which the resonator 3 or 4 vibrates in anenclosed manner. In the first preferred embodiment, although the spacers51 to 54 are attached to the corresponding sides of the base plate 5 andare separated by a predetermined distance in the longitudinal directionof the base plate 5 in order to form the gaps 5 a, the opposite sides ofthe base plate 5 may alternatively be provided with depressions in placeof the spacers. As a further alternative, the base plate 5 and each ofthe resonators 3 and 4 may have an adhesive layer therebetween havingenough thickness for forming gaps.

The spacers 51 and 52 disposed adjacent to the base end have the samelength as the spacers 31 and 41 disposed adjacent to the base ends ofthe respective resonators 3 and 4. Moreover, the height of the spacers51 and 52 (in the z-axis direction) is preferably substantially the sameas the height H₂ of the base plate 5. Similarly, the spacers 53 and 54disposed adjacent to the free end have the same length as the spacers 32and 42 disposed adjacent to the free ends of the respective resonators 3and 4. Moreover, the height of the spacers 53 and 54 (in the z-axisdirection) is preferably substantially the same as the height H₂ of thebase plate 5.

The resonators 3 and 4, the spacers 31, 32, 41, and 42, the base plate5, the spacers 51 to 54 define the acceleration-sensor element 2A andare composed of materials having the same coefficient of thermalexpansion as that of the resonators 3 and 4 (for example, a ceramicmaterial such as PZT). This prevents stress from being generated in theresonators 3 and 4 due to differences in thermal expansion caused by achange in temperature.

One side surface of the base plate 5 having the spacers 51 and 53attached thereon is provided with an extraction electrode 5 b extendingover the entire length of the side surface. The extraction electrode 5 bis connected with an internal electrode 61 extending continuously acrossthe top surface of the base-end portion of the acceleration-sensorelement 2A when the resonators 3 and 4 are in a combined state. On theother hand, an internal electrode 64 extends continuously across afree-end portion of the top surface of the combination of the base plate5 and the spacers 53, 54, 32, and 42. The internal electrode 64functions as a connector for interconnecting the extraction electrode 5b disposed on one side surface of the base plate 5 with the connectionelectrodes 34 and 44 disposed on the side surfaces of the respectiveresonators 3 and 4.

The two opposite sides of the acceleration-sensor element 2A withrespect to the application direction of acceleration G are respectivelycovered with the pair of left and right casing components 6. Each casingcomponent 6 is preferably substantially U-shaped in cross section, and aprojection 6 a disposed adjacent to a first end of the casing component6 is attached to the base-end portion of one of the opposite sidesurfaces of the acceleration-sensor element 2A. On the other hand, aprojection 6 b disposed adjacent to a second end of one casing component6 is attached to a projection 6 b of the other casing component 6 via aspacer 2 a disposed therebetween. The spacer 2 a according to the firstpreferred embodiment is formed by cutting a longitudinal end-segment ofthe acceleration-sensor element 2A, and includes portions of the baseplate 5, the resonators 3 and 4, and the spacers 53, 54, 32, and 42. Theprojections 6 a and 6 b of each casing component 6 have a depression 6 cdisposed therebetween, which is a space where the acceleration-sensorelement 2A is allowed to bend into. Moreover, each casing component 6 isprovided with a stopper 6 d disposed near an inner side of thesecond-end projection 6 b. The stopper 6 d restricts anover-displacement of the acceleration-sensor element 2A when a largeamount of acceleration G is applied so as to prevent theacceleration-sensor element 2A from being deformed or damaging. If thedegree of bending of the acceleration-sensor element 2A is extremelysmall and the bending spaces can thus be formed based on the thicknessof adhesive layers between the casing components 6 and theacceleration-sensor element 2A, the depressions 6 c and the stoppers 6 dmay be omitted.

The inner side surface and the top surface of one casing component 6 arerespectively provided with extraction electrodes 62 a and 62 b which areconnected with each other, and the inner side surface and the topsurface of the other casing component 6 are respectively provided withextraction electrodes 63 a and 63 b which are connected with each other.The casing components 6 are joined with the acceleration-sensor element2A via an electrically conductive adhesive for allowing the electrodes33 and 62 a to be electrically connected with each other, and theelectrodes 43 and 63 a to be electrically connected with each other. Inthis case, an anisotropic electrically-conductive adhesive is used inorder to prevent a short circuit between the internal electrode 61,extending continuously across the base-end portion of the top surface ofthe combination of the casing components 6 and the acceleration-sensorelement 2A, and an external electrode 71, and between the electrode 33and an electrode 4 c.

The extraction electrodes 62 b and 63 b disposed on the top surfaces ofthe corresponding casing components 6 are aligned with the internalelectrode 64 disposed on the free-end portion of the top surface of theacceleration-sensor element 2A. The electrodes 62 b, 63 b, and 64 areformed after the casing components 6 are attached to theacceleration-sensor element 2A, and can be fabricated simultaneously byperforming, for example, a sputtering process or a deposition process onthe top surface of the combination of the casing components 6 and theacceleration-sensor element 2A. In this case, the internal electrode 61can also be formed at the same time.

The upper and lower open planes of the combination of theacceleration-sensor element 2A and the casing components 6 arerespectively covered with the pair of upper and lower cover components7. The inner surface of each cover component 7 is provided with acavity-forming recess 7 a for preventing the acceleration-sensor element2A from coming into contact with the cover component 7. A peripheralregion surrounding the recess 7 a is attached to one of the open planes.For this reason, a portion of the acceleration-sensor element 2A to bedisplaced in response to acceleration G is completely enclosed by thecasing components 6 and the cover components 7. Similar to the casingcomponents 6, the cavity-forming recess 7 a in the inner surface of eachcover component 7 may be omitted if the cavity can be formed based onthe thickness of an adhesive layer provided along the frame region onthe inner surface of the cover component 7.

The outer surface of each cover component 7 is provided with a portionof an external electrode 71 positioned adjacent to the base end of theacceleration-sensor element 2A, and portions of two external electrodes72 and 73 positioned close to the free end of the acceleration-sensorelement 2A. Referring to FIG. 1, the two external electrodes 72 and 73are positioned distant from the external electrode 71 in thelongitudinal direction (x-axis direction), and moreover, are disposed ontwo opposite sides from each other in the application direction ofacceleration (y-axis direction). The positioning of the two externalelectrodes 72 and 73 is not limited to that shown in FIG. 1.Alternatively, the external electrodes 72 and 73 may be disposed at anend opposite to the end at which the external electrode 71 is disposed,such that the two electrodes 72 and 73 are disposed on opposite sides inthe y-axis direction at that end.

The acceleration sensor 1A having the structure described above has thefollowing conductive path.

Specifically, the upper electrode 3 a of the resonator 3 is connectedwith the external electrode 72 via the connection electrode 33 and theextraction electrodes 62 a and 62 b. On the other hand, the upperelectrode 4 a of the resonator 4 is connected with the externalelectrode 73 via the connection electrode 43 and the extractionelectrodes 63 a and 63 b. The lower electrodes 3 b and 4 b of therespective resonators 3 and 4 are interconnected with each other via theconnection electrodes 34 and 44 and the internal electrode 64, and areconnected with the external electrode 71 via the extraction electrode 5b disposed on one side surface of the base plate 5, and the internalelectrode 61.

Although only one extraction electrode 5 b is provided on one sidesurface of the base plate 5, two extraction electrodes 5 b mayalternatively be provided on the two opposite side surfaces of the baseplate 5. This may contribute to a further prevention of disconnection ofthe conductive path.

Accordingly, a surface-mounted-chip acceleration sensor 1A is obtained.

FIG. 6 is a circuit diagram of an example of an acceleration sensordevice provided with the acceleration sensor 1A.

Such a sensor device utilizes separate oscillation effects of theacceleration-sensor element 2A. Specifically, the external electrodes 72and 71 of the acceleration sensor 1A are connected with an oscillationcircuit 9 a, and the external electrodes 73 and 71 are connected with anoscillation circuit 9 b. Each of the oscillation circuits 9 a and 9 bmay be, for example, the commonly known Colpitts oscillation circuit.The resonators 3 and 4 are separately oscillated by the respectiveoscillation circuits 9 a and 9 b. The oscillating frequencies f₁ and f₂are then input to a frequency-difference counter 9 c. Subsequently, thefrequency-difference counter 9 c outputs a signal V₀, which isproportional to the difference in the frequencies.

When acceleration G is applied to the acceleration sensor 1A, an inertiaforce acts upon the acceleration-sensor element 2A in a directionopposite to the direction in which the acceleration is applied. Thisbends the acceleration-sensor element 2A in the opposite direction tothe application direction of acceleration G. The bending of theacceleration-sensor element 2A generates stress, thus producing tensilestress in one of the resonators and compressive stress in the otherresonator. In the case where resonators of a thickness-shear vibrationtype are used, the oscillating frequency of the resonator with tensilestress decreases, whereas the oscillating frequency of the resonatorwith compressive stress increases. Accordingly, the difference in thefrequencies is obtained via the external electrodes 71, 72, and 73 sothat the signal V₀ proportional to the acceleration G can be obtained.

Using the acceleration sensor 1A in an environment where there is achange in temperature may lead to thermal expansion of the resonators 3and 4, the base plate 5, the casing components 6, and the covercomponents 7. If the coefficient of thermal expansion is different amongthe resonators 3 and 4 and the base plate 5, the acceleration-sensorelement 2A may bend when the temperature changes, thus generating stressin the resonators 3 and 4. This means that the difference in thefrequencies may change due to factors other than acceleration. On theother hand, if the resonators 3 and 4 and the base plate 5 are composedof materials having substantially the same coefficient of thermalexpansion, the same amount of stress will be generated in response to achange in temperature. Consequently, the outputs from the two resonators3 and 4 are received by the frequency-difference counter 9 c in adifferential manner, such that the changes in the output signals causedby, for example, a change in temperature affecting both resonators 3 and4 can be counterbalanced. Accordingly, an acceleration sensor devicehaving sensitivity solely against acceleration G can be obtained.

On other hand, even if the coefficient of thermal expansion among theacceleration-sensor element 2A, the casing components 6, and the covercomponents 7 is different, a temperature change simply does not lead toa generation of stress in the acceleration-sensor element 2A since theacceleration-sensor element 2A is supported by these components only ina cantilever manner.

FIG. 7 illustrates another example of an acceleration sensor deviceprovided with the acceleration sensor 1A.

This acceleration sensor device utilizes a single oscillation effect ofthe acceleration-sensor element 2A. Specifically, the externalelectrodes 72 and 73 of the acceleration sensor 1A are connected with adifferential impedance sensor circuit 9 d, and the external electrode71, which is a common electrode, is connected with an oscillationcircuit 9 e. Moreover, reference numerals 9 f and 9 g indicate matchingresistors. In this case, both resonators 3 and 4 are oscillated with thesame frequency by the oscillation circuit 9 e, so that the phasedifference or the oscillation difference can be detected based on thedifference in electrical impedance between the resonators 3 and 4. Thus,the output V₀ proportional to acceleration G is obtained via thedifferential impedance sensor circuit 9 d. In order to achieveoscillation with the same frequency, the oscillator circuit 9 e may beformed in view of feedback on an output from one of the resonators or acombination of outputs from both resonators.

In this case, like the example shown in FIG. 6, a signal proportional toacceleration G can be obtained while also counterbalancing the changesin the outputs caused by, for example, a change in temperature.Accordingly, an acceleration sensor device having sensitivity that isreactive only to acceleration G can be obtained.

Second Preferred Embodiment

FIGS. 8 to 10 illustrate an acceleration sensor according to a secondpreferred embodiment.

An acceleration sensor 1B is different from the first preferredembodiment in that electrodes 3 c and 3 d of a resonator 3B andelectrodes 4 c and 4 d of a resonator 4B in a bimorphacceleration-sensor element 2B are disposed on main surfaces which aresubstantially perpendicular to the application direction of accelerationG. Components equivalent to those in the first preferred embodiment areindicated by the same reference numerals, and descriptions of thosecomponents will thus be omitted.

The electrodes 3 c and 4 c of the respective resonators 3B and 4B areexposed at the outer surfaces of the acceleration-sensor element 2B. Onthe other hand, the electrodes 3 d and 4 d face the base plate 5. Afirst-end portion of the electrode 3 c on one surface of the resonator3B is opposed to a second-end portion of the electrode 3 d on the othersurface of the resonator 3B at an intermediate portion of the resonator3B with respect to the longitudinal direction thereof, and similarly, afirst-end portion of the electrode 4 c on one surface of the resonator4B is opposed to a second-end portion of the electrode 4 d on the othersurface of the resonator 4B at an intermediate portion of the resonator4B with respect to the longitudinal direction thereof. On the otherhand, second and first-end portions of the respective electrodes 3 c and3 d do not completely extend to the corresponding ends of the resonator3B, and similarly, second and first-end portions of the respectiveelectrodes 4 c and 4 d do not completely extend to the correspondingends of the resonator 4B. The resonators 3B and 4B preferably havesubstantially the same height H₁ in the z-axis direction, and preferablyhave substantially the same thickness T₁ in the y-axis direction. Thus,the resonators 3B and 4B have the same resonance frequency when noacceleration is being applied thereto. Since the height H₁ of theresonators 3B and 4B is smaller than the height H₂ of the base plate 5in the z-axis direction, the stress generated in the resonators 3B and4B when acceleration is being applied thereto is greater than in a casewhere H₁=H₂.

The vibrating section E in each of the resonators 3B and 4B ispositioned closer to the fixed end. Specifically, the distance L1 fromthe vibrating section E to the fixed end is shorter than the distance L2from the vibrating section E to the free end. Accordingly, anacceleration sensor having high sensitivity, in which signals can beobtained via sections where a large bending stress is generated inresponse to acceleration G, is provided.

The connection electrodes 33 and 43 disposed close to the base ends ofthe resonators 3B and 4B, respectively, are disposed on the outer sidesurface of the combination of the resonator 3B and the spacers 31 andthe outer side surface of the combination of the resonator 4B and thespacers 41. Thus, the connection electrode 33 is surface-connected withthe electrode 3 c of the resonator 3B, and the connection electrode 34is surface-connected with the electrode 3 d of the resonator 3B.Similarly, the connection electrode 43 is surface-connected with theelectrode 4 c of the resonator 4B, and the connection electrode 44 issurface-connected with the electrode 4 d of the resonator 4B.Accordingly, this ensures the electrical connections among thecomponents.

In the acceleration sensor 1B according to the second preferredembodiment, the electrodes 3 c and 3 d of the resonator 3B and theelectrodes 4 c and 4 d of the resonator 4B are not exposed at the upperand lower sides of the acceleration-sensor element 2B. For this reason,unlike the first preferred embodiment, the electrodes in the secondpreferred embodiment cannot be trimmed by using, for example, laser, orbe coated with, for example, frequency-regulating ink after the casingcomponents 6 are joined with the acceleration-sensor element 2B.However, since the structure of the second preferred embodiment ensuresthe connections between the connection electrodes 33 and 34 and therespective electrodes 3 c and 3 d of the resonator 3B, and theconnections between the connection electrodes 43 and 44 and therespective electrodes 4 c and 4 d of the resonator 4B, an accelerationsensor with a high reliability can be obtained.

Third Preferred Embodiment

FIGS. 11 and 12 illustrate an acceleration sensor according to a thirdpreferred embodiment.

An acceleration sensor 1C is similar to the second preferred embodimentin that electrodes 3 e and 3 f of a resonator 3C and electrodes 4 e and4 f of a resonator 4C in a bimorph acceleration-sensor element 2C aredisposed on main surfaces which are substantially perpendicular to theapplication direction of acceleration G. On the other hand, theacceleration sensor 1C is different from the second preferred embodimentin that the height H₁ of the resonators 3C and 4C is preferablysubstantially the same as the height H₂ of the base plate 5. Componentsequivalent to those in the first and second preferred embodiments areindicated by the same reference numerals, and descriptions of thosecomponents will thus be omitted.

Since the height H₁ of the resonators 3C and 4C is substantially thesame as that of the base plate 5, unlike the first and second preferredembodiments, the spacers 31 and 32 and the spacers 41 and 42 areomitted. The vibrating section E in each of the resonators 3C and 4C ispositioned closer to the fixed end. Specifically, the distance L1 fromthe vibrating section E to the fixed end is shorter than the distance L2from the vibrating section E to the free end. Accordingly, anacceleration sensor having high sensitivity, in which signals can beobtained via sections where a large bending stress is generated inresponse to acceleration G, is provided.

In this case, since the electrodes 3 e and 3 f and the electrodes 4 eand 4 f disposed on side surfaces of the acceleration-sensor element 2Calso function as connection electrodes, the fabrication process for theelectrodes can be simplified.

The acceleration sensor according to the present invention is notlimited to the above-described preferred embodiments.

For example, although the resonators used in the first and secondpreferred embodiments are of a thickness-shear vibration type,resonators of other alternative vibration types (such as athickness-extensional vibration type or a longitudinal vibration type)may be used.

Furthermore, although the base plate and each of the first and secondresonators in the above-described preferred embodiments have a gaptherebetween given a wider dimension than the range in which theresonator vibrates in an enclosed manner, the base plate and eachresonator may alternatively be joined with each other in an opposingmanner such that the surfaces of the base plate and the resonator areentirely attached to each other. Such an entirely-attached state maycause deterioration of the performance (Q and K) of the resonators sincethe base plate limits the vibration of the resonators, but is effectivein view of the efficiency for generating stress in response to theacceleration.

Although the present invention has been described and illustrated indetail with reference to certain preferred embodiments thereof, it isclearly understood that the same is by way of illustration and exampleonly and is not to be taken by way of limitation, the spirit and scopeof the present invention being limited only by the terms of the appendedclaims.

1-7. (canceled)
 8. An acceleration sensor comprising: a base plate; and first and second resonators each including a piezoelectric material and each having electrodes on two opposite main surfaces thereof, each of the first and second resonators having a vibrating section at an intermediate portion of the resonator with respect to the longitudinal direction thereof; wherein the first and second resonators are attached to opposite sides of the base plate with respect to a direction in which acceleration is applied so as to define a bimorph acceleration-sensor element, one longitudinal end of the acceleration-sensor element is fixed such that the first and second resonators bend in the same direction in response to the acceleration, and changes in frequency or changes in impedance in the first and second resonators caused by the bending of the acceleration-sensor element are differentially detected in order to detect the acceleration, the acceleration-sensor element is bendable about a central bending plane in response to the acceleration, the central bending plane being positioned at a central portion of the base plate with respect to the application direction of acceleration, and the vibrating section of each of the first and second resonators is disposed close to the fixed end of the acceleration-sensor element.
 9. The acceleration sensor according to claim 8, wherein a height of the first and second resonators in a direction that is substantially perpendicular to the application direction of acceleration is smaller than a height of the base plate in the direction that is substantially perpendicular to the application direction of acceleration.
 10. The acceleration sensor according to claim 9, wherein the first and second resonators are attached to the opposite sides of the base plate at positions where the first and second resonators are opposed to each other.
 11. The acceleration sensor according to claim 10, wherein each of the first and second resonators is attached to the central portion of the base plate with respect to a height direction of the base plate, the height direction being substantially perpendicular to the application direction of acceleration.
 12. The acceleration sensor according to claim 8, wherein the base plate and the first and second resonators are made of at least one material having substantially the same coefficient of thermal expansion.
 13. The acceleration sensor according to claim 8, wherein opposite outer surfaces of the acceleration-sensor element are respectively fixedly supported by a pair of casing components at said longitudinal end of the acceleration-sensor element, the outer surfaces being opposite to each other with respect to the application direction of acceleration, and open planes defined by the acceleration-sensor element and the casing components are covered with a pair of cover components so that a displacement portion of the acceleration-sensor element, which is bendable in response to the acceleration, is disposed within an enclosed space, one of the electrodes in each of the first and second resonators is disposed at a free-end side of the resonator and is connected with a common electrode via an extraction electrode provided on the base plate, the common electrode being provided at a fixed-end side of an outer surface of a combination of the casing components and the cover components, the other electrode in the first resonator is disposed at a base-end side of the first resonator, said electrode being connected with a first independent electrode provided at a free-end side of the outer surface of the combination of the casing components and the cover components, said electrode being connected with the first independent electrode via a first extraction electrode provided on one of the casing components, and the other electrode in the second resonator is disposed at a base-end side of the second resonator, said electrode being connected with a second independent electrode provided at the free-end side of the outer surface of the combination of the casing components and the cover components, said electrode being connected with the second independent electrode via a second extraction electrode provided on the other casing component.
 14. The acceleration sensor according to claim 8, wherein the casing components are provided with a plurality of internal electrodes disposed on upper surfaces of the casing components, the internal electrodes being connected with the electrodes in each of the first and second resonators. 